This invention relates to on-chip testing circuits. More specifically, this invention relates to on-chip substrate voltage regulators for use during defect testing.
During testing for margin defects in packaged semiconductor integrated circuit chips, it is desirable to vary the voltage level of the substrate from its normal negative operating level. One method is to set the substrate voltage level to ground. However, setting the substrate voltage level to ground during testing of some types of chips, such as 16-megabyte memory chips, may be an unrealistic testing condition because some chips that fail the testing process would operate satisfactorily with a negatively biased substrate. What is needed is an on-chip substrate regulator with the ability to vary the substrate voltage level during testing to be more positive or more negative than its normal negative operating level while maintaining the substrate voltage level below ground.
An on-chip circuit provides the ability to maintain a substrate voltage at a level more positive or more negative than a normal negative operating voltage level of the substrate. This is accomplished with a chain of MOSFETs that are coupled to operate as a chain of resistive elements or diodes wherein each element in the chain may drop a portion of a supply voltage coupled to a first end the chain. The chain is nominatively referred to as a diode chain. The substrate is coupled to a second end of the diode chain. The substrate voltage level is equivalent to the supply voltage level less the voltage drops across the elements in the diode chain. A charge pump maintains the substrate at the voltage level set by the diode chain.
A first plurality of MOSFETs in the diode chain are configured to be normally shorted. When these MOSFETs are controlled to change from a shorted condition to a condition of operating as diodes or resistive elements, the substrate level becomes more negative due to the added voltage drop. A second plurality of MOSFETs in the diode chain are configured to operate normally as diodes or resistive elements. When these MOSFETs are controlled to change from operating as diodes or resistive elements to a shorted condition, the substrate level becomes more positive due to the removed voltage drop. A third plurality of MOSFETs are coupled as switches to control whether the MOSFETs in the first and second pluralities of MOSFETs are shorted or are operating as diodes when it is desired to vary the substrate voltage level during testing.
Performing chip testing with the substrate voltage level more negative than the normal negative voltage level facilitates detection of devices that will tend to fail only at cold temperatures. Performing chip testing with the substrate voltage level more positive than the normal negative voltage level facilitates detection of other margin failures and ion contamination.